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          2020
        </a></span></div></div> <div data-v-3ba18f14><main class="page"><div class="page-title" style="display:none;"><h1 class="title"></h1> <div data-v-5d8dbdb4><i class="iconfont reco-account" data-v-5d8dbdb4><span data-v-5d8dbdb4>海星</span></i> <!----> <!----> <!----></div></div> <div class="theme-reco-content content__default" style="display:none;"><blockquote><p>面试官：带笔了吧，那写两种单例模式的实现方法吧</p> <p>沙沙沙刷刷刷~~~ 写好了</p> <p>面试官：你这个是怎么保证线程安全的，那你知道，volatile 关键字? 类加载器？锁机制？？？？
点赞+收藏 就学会系列，文章收录在 GitHub <a href="https://github.com/Jstarfish/JavaEgg" target="_blank" rel="noopener noreferrer">JavaEgg<svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg></a> ，N线互联网开发必备技能兵器谱</p></blockquote> <h2 id="单例模式-独一无二的对象"><a href="#单例模式-独一无二的对象" class="header-anchor">#</a> 单例模式——独一无二的对象</h2> <p><img src="https://i01piccdn.sogoucdn.com/d4a728c10d74ab67" alt=""></p> <p>单例模式，从我看 《Java 10分钟入门》那天就听过的一个设计模式，还被面试过好几次的设计模式问题，今天一网打尽~~</p> <p>有一些对象我们确实只需要一个，比如，线程池、数据库连接、缓存、日志对象等，如果有多个的话，会造成程序的行为异常，资源使用过量或者不一致的问题。你也许会说，这种我用全局变量不也能实现吗，还整个单例模式，好像你很流弊的样子，如果将对象赋值给一个全局变量，那程序启动就会创建好对象，万一这个对象很耗资源，我们还可能在某些时候用不到，这就造成了资源的浪费，不合理，所以就有了单例模式。</p> <h2 id="单例模式的定义"><a href="#单例模式的定义" class="header-anchor">#</a> 单例模式的定义</h2> <p><strong>单例模式确保一个类只有一个实例，并提供一个全局唯一访问点</strong></p> <h2 id="单例模式的类图"><a href="#单例模式的类图" class="header-anchor">#</a> 单例模式的类图</h2> <p><img src="https://tva1.sinaimg.cn/large/006tNbRwly1gbjeonzilrj309u064t93.jpg" alt=""></p> <h2 id="单例模式的实现"><a href="#单例模式的实现" class="header-anchor">#</a> 单例模式的实现</h2> <h3 id="饿汉式"><a href="#饿汉式" class="header-anchor">#</a> 饿汉式</h3> <ul><li>static 变量在类装载的时候进行初始化</li> <li>多个实例的 static 变量会共享同一块内存区域</li></ul> <p>用这两个知识点写出的单例类就是饿汉式了，初始化类的时候就创建，饥不择食，饿汉</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>

    <span class="token comment">//构造私有化，防止直接new</span>
    <span class="token keyword">private</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

    <span class="token comment">//静态初始化器（static initializer）中创建实例，保证线程安全</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> instance<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>饿汉式是线程安全的，JVM在加载类时马上创建唯一的实例对象，且只会装载一次。</p> <p>Java 实现的单例是一个虚拟机的范围，因为装载类的功能是虚拟机的，所以一个虚拟机通过自己的ClassLoader 装载饿汉式实现单例类的时候就会创建一个类实例。（如果一个虚拟机里有多个ClassLoader的话，就会有多个实例）</p> <h3 id="懒汉式"><a href="#懒汉式" class="header-anchor">#</a> 懒汉式</h3> <p>懒汉式，就是实例在用到的时候才去创建，比较“懒”</p> <p>单例模式的懒汉式实现方式体现了延迟加载的思想（延迟加载也称懒加载Lazy Load，就是一开始不要加载资源或数据，等到要使用的时候才加载）</p> <h4 id="同步方法"><a href="#同步方法" class="header-anchor">#</a> 同步方法</h4> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> singleton<span class="token punctuation">;</span>

    <span class="token keyword">private</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

  	<span class="token comment">//解决了线程不安全问题，但是效率太低了，每个线程想获得类的实例的时候，都需要同步方法，不推荐</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">synchronized</span> <span class="token class-name">Singleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>singleton <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            singleton <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> singleton<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h4 id="双重检查加锁"><a href="#双重检查加锁" class="header-anchor">#</a> 双重检查加锁</h4> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>

  	<span class="token comment">//volatitle关键词确保，多线程正确处理singleton</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">volatile</span> <span class="token class-name">Singleton</span> singleton<span class="token punctuation">;</span>
  
    <span class="token keyword">private</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>
  
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>singleton <span class="token operator">==</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token class-name">Singleton</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token keyword">if</span><span class="token punctuation">(</span>singleton <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                    singleton <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> singleton<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Double-Check 概念（进行两次检查）是多线程开发中经常使用的，为什么需要双重检查锁呢？因为第一次检查是确保之前是一个空对象，而非空对象就不需要同步了，空对象的线程然后进入同步代码块，如果不加第二次空对象检查，两个线程同时获取同步代码块，一个线程进入同步代码块，另一个线程就会等待，而这两个线程就会创建两个实例化对象，所以需要在线程进入同步代码块后再次进行空对象检查，才能确保只创建一个实例化对象。</p> <p>双重检查加锁（double checked locking）线程安全、延迟加载、效率比较高</p> <p><strong>volatile</strong>：volatile一般用于多线程的可见性，这里用来防止<strong>指令重排</strong>（防止new Singleton时指令重排序导致其他线程获取到未初始化完的对象）。被volatile 修饰的变量的值，将不会被本地线程缓存，所有对该变量的读写都是直接操作共享内存，从而确保多个线程能正确的处理该变量。</p> <h5 id="指令重排"><a href="#指令重排" class="header-anchor">#</a> 指令重排</h5> <p>指令重排是指在程序执行过程中, 为了性能考虑, 编译器和CPU可能会对指令重新排序。</p> <p>Java中创建一个对象，往往包含三个过程。对于singleton = new Singleton()，这不是一个原子操作，在 JVM 中包含如下三个过程。</p> <ol><li><p>给 singleton 分配内存</p></li> <li><p>调用 Singleton 的构造函数来初始化成员变量，形成实例</p></li> <li><p>将singleton对象指向分配的内存空间（执行完这步 singleton才是非 null 了）</p></li></ol> <p>但是，由于JVM会进行指令重排序，所以上面的第二步和第三步的顺序是不能保证的，最终的执行顺序可能是 1-2-3，也可能是 1-3-2。如果是 1-3-2，则在 3 执行完毕，2 未执行之前，被另一个线程抢占了，这时 instance 已经是非 null 了（但却没有初始化），所以这个线程会直接返回 instance，然后使用，那肯定就会报错了，所以要加入 volatile关键字。</p> <h3 id="静态内部类"><a href="#静态内部类" class="header-anchor">#</a> 静态内部类</h3> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>

    <span class="token keyword">private</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">SingletonInstance</span><span class="token punctuation">{</span>
        <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">Singleton</span> INSTANCE <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token class-name">SingletonInstance</span><span class="token punctuation">.</span>INSTANCE<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>采用类加载的机制来保证初始化实例时只有一个线程；</p> <p>静态内部类方式在Singleton 类被装载的时候并不会立即实例化，而是在调用getInstance的时候，才去装载内部类SingletonInstance ,从而完成Singleton的实例化</p> <p>类的静态属性只会在第一次加载类的时候初始化，所以，JVM帮我们保证了线程的安全性，在类初始化时，其他线程无法进入</p> <p>优点：线程安全，利用静态内部类实现延迟加载，效率较高，推荐使用</p> <h3 id="枚举"><a href="#枚举" class="header-anchor">#</a> 枚举</h3> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">enum</span> <span class="token class-name">Singleton</span><span class="token punctuation">{</span>
  INSTANCE<span class="token punctuation">;</span>
  <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>借助JDK5 添加的枚举实现单例，不仅可以避免多线程同步问题，还能防止反序列化重新创建新的对象，但是在枚举中的其他任何方法的线程安全由程序员自己负责。还有防止上面的通过反射机制调用私用构造器。不过，由于Java1.5中才加入enum特性，所以使用的人并不多。</p> <p>这种方式是《Effective Java》 作者Josh Bloch 提倡的方式。</p> <h2 id="单例模式在jdk-中的源码分析"><a href="#单例模式在jdk-中的源码分析" class="header-anchor">#</a> 单例模式在JDK 中的源码分析</h2> <p>JDK 中，<code>java.lang.Runtime</code> 就是经典的单例模式（饿汉式）</p> <p><img src="https://tva1.sinaimg.cn/large/006tNbRwly1gbig3eaoe8j31al0u0qg5.jpg" alt=""></p> <h2 id="单例模式注意事项和细节"><a href="#单例模式注意事项和细节" class="header-anchor">#</a> 单例模式注意事项和细节</h2> <ul><li>单例模式保证了系统内存中该类只存在一个对象，节省了系统资源，对于一些需要频繁创建销毁的对象，使用单例模式可以提高系统性能</li> <li>当想实例化一个单例类的时候，必须要记住使用相应的获取对象的方法，而不是使 用new</li> <li>单例模式使用的场景：需要频繁的进行创建和销毁的对象、创建对象时耗时过多或 耗费资源过多(即:重量级对象)，但又经常用到的对象、工具类对象、频繁访问数 据库或文件的对象(比如数据源、session工厂等)</li></ul></div> <footer class="page-edit" style="display:none;"><!----> <!----></footer> <!----> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-db14854a data-v-db14854a><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-db14854a><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-db14854a></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-db14854a></path></svg></div><!----></div></div>
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